Doubly-fed electric machines have windings on both stationary and rotating parts, where both windings transfer significant power between a shaft and an electrical system. Doubly-fed machines are useful in applications that specify a varying speed of the machine's shaft for a fixed power system frequency. Doubly-fed generators are, for example, widely used in wind turbines.
A doubly-fed generator can include a frequency controller connected to a rotor circuit. As the power through the rotor windings depends on the slip frequency, a frequency converter can be rated according to a maximum power through the rotor windings. If a range for the slip frequency is limited, the maximum power through the rotor windings may be only a fraction of the total power generated by the generator.
One way to control a doubly-fed electric generator is to use a torque and flux controller, for example as disclosed in patent publication U.S. Pat. No. 6,448,735 B1. A torque and flux controller, such as a Direct Torque Control (DTC) controller, uses a determined torque and flux to choose one of eight voltage vectors, which, in the case of doubly fed generator, is used to steer the rotor flux.
One of the disadvantages associated with these controllers is that the grid supplied by the generator can be assumed to be balanced. In practice there are many situations when the grid is more or less unbalanced. For instance, asymmetry of the loads can cause unbalance. Abnormal system conditions, such as phase-to-ground, phase-to-phase and open-conductor faults, can also cause phase unbalance. Since the generator stator is directly connected to the grid, unbalance in grid voltage causes large oscillations in currents which, in turn, can complicate the control of the generator.
Specification of maximum ratings of a doubly-fed generator can be carried out based on a winding ratio of the stator and the rotor. For example, in wind turbine applications, the winding ratio can be 1:2.6, for example, and the nominal voltage for the stator is, for example, 690 V. When the generator is at a standstill and the slip frequency is 50 Hz, for example, the generator rotor and stator act as a transformer, and the rotor voltage is (2.6·690 V=) 1800 V, thus giving a voltage rating of 1800 V.
A magnitude of the rotor voltage is proportional to the slip frequency, so the operational range in doubly-fed generator applications can be restricted to ±30% of the nominal speed.
At nominal speed, the unbalance can cause oscillations, for example, at a frequency of 100 Hz, to the rotor voltage. Since the magnitude of the rotor voltage is proportional to the slip frequency, and since the slip frequency is now 100 Hz, the voltage magnitude on the stator side, caused by unbalance, has to be only half (345 V) of the nominal voltage in order for the rotor voltage to reach the limit of 1800 V. The voltage oscillations cause current spikes in the stator and rotor circuit, because the generator flux is not able to follow changes in the grid voltage.
In general, the unbalance can cause reduction in useful torque, mechanical damage to bearings and faster thermal aging through excessive heating. Prior art control methods operating based on an assumption that the grid is balanced can aggravate these problems.